1. Field of the Invention
The subject matter of the present invention relates to thermoplastic composite materials and prepregs used for creating structures that have a high strength-to-weight ratio. More particularly, in certain embodiments the present invention relates to layered thermoplastic composites for use in rapid lamination and forming processes, which composites possess an interlayer region containing at least one polymer that is high in crystallinity and either partially or fully crystallizes during the process window of the rapid lamination or forming process, and an outerlayer region containing a polymer that has low or no crystallization and is miscible and/or compatible with the polymer of the interlayer region, and has a lower melting and processing temperature than the polymer of the interlayer region. Such thermoplastic composites are useful in aerospace and other high-performance automotive/industrial applications.
2. Description of the Related Art
Reinforced thermoplastic and thermoset materials have wide application in, for example, the aerospace, automotive, industrial/chemical, and sporting goods industries. Thermosetting resins are impregnated into the reinforcing material before curing, while the resinous materials are low in viscosity. Thermoset composites suffer from several disadvantages. Low molding pressures are used to prepare these composites to avoid damage to the fibers. These low pressures, however, make it difficult to suppress the formation of bubbles within the composite which can result in voids or defects in the matrix coating. Thus, most processing problems with thermoset composites are concerned with removing entrained air or volatiles so that a void-free matrix is produced. Thermoset composites made by the prepreg method require lengthy cure times with alternating pressures to control the flow of the resin as it thickens to prevent bubbles in the matrix. While current fabrication of large structures utilize robotic placement of the thermoset composite material to increase production rate, its overall production rate for the component is limited by the lengthy cure in the autoclave process step and related operations to prepare the material for that process step. Some high volume processes, such as resin infusion avoid the prepreg step but still require special equipment and materials along with constant monitoring of the process over the length of the cure time (e.g. U.S. Pat. Nos. 4,132,755, and 5,721,034). Although thermoset resins have enjoyed success as in lower volume composites applications, the difficulties in processing these resins has limited their use in larger volume applications.
Thermoplastic compositions, in contrast, are more difficult to impregnate into the reinforcing material because of comparatively higher viscosities. On the other hand, thermoplastic compositions offer a number of benefits over thermosetting compositions. For example, thermoplastic prepregs can be more rapidly fabricated into articles. Another advantage is that thermoplastic articles formed from such prepregs may be recycled. In addition, damage resistant composites with excellent performance in hot humid environments may be achieved by the proper selection of the thermoplastic matrix. Thermoplastic resins are long chain polymers of high molecular weight. These polymers are highly viscous when melted and are often non-Newtonian in their flow behavior. Thus, whereas thermosets have viscosities in the range of 100 to 5,000 centipoise (0.1 to 5 Pa*s), thermoplastics have melt viscosities ranging from 5,000 to 20,000,000 centipoise (5 to 20,000 Pa*s), and more typically from 20,000 to 1,000,000 centipoise (20 to 1000 Pa*s). Despite a viscosity difference of three orders of magnitude between thermosets and thermoplastics, some processes have been applied to both types of matrices for laminating fibrous materials.
Fiber-reinforced plastic materials may be manufactured by first impregnating the fiber reinforcement with resin to form a prepreg, then consolidating two or more prepregs into a laminate, optionally with additional forming steps. Due to the high viscosity of thermoplastics, most of the processes to form thermoplastic prepregs involve coating the outside of the fiber bundles with a thermoplastic polymer powder rather than coating individual filaments. The polymer powder is then melted to force the polymer around, into and onto the individual filaments. A few processes apply melt directly to the fibers. A tape can be made by coating a dry web of collimated fibers with the polymer and applying a heated process that forces the polymer into and around the fibers (e.g., see U.S. Pat. Nos. 4,549,920 and 4,559,262). Another process used to coat and impregnate a dry web of collimated fibers is by pulling the web through an aqueous slurry of fine thermoplastic polymer particles whereby the polymer particles are trapped within the filament bundles. Subsequent heat and pressure in the process boils off the water and then melts the polymer to force it into and around the filament bundles. This process is described in U.S. Pat. Nos. 6,372,294; 5,725,710; 4,883,552 and 4,792,481. A modification to the aqueous slurry impregnation process is to eliminate the use of water and surfactant as dispersing agents for the polymer particles and instead electrostatically charge the particles in a fluidized bed of air to trap the particles in the filament bundle. Subsequent zones of heat and pressure melt the polymer to coat/impregnate the filament bundle as given in U.S. Pat. No. 5,094,883. Thus, for those skilled in the art, there are multiple methods to coat and/or impregnate a fibrous substrate given the available process equipment, and proper selection of polymer product form (flake, fine powder, film, non-woven veil, pellets) and melt viscosity. Known methods for the fabrication of composite articles include manual and automated fabrication. Manual fabrication entails manual cutting and placement of material by a technician to a surface of the mandrel. This method of fabrication is time consuming and cost intensive, and could possibly result in non-uniformity in the lay-up. Known automated fabrication techniques include: flat tape laminating machines (FTLM) and contour tape laminating machines (CTLM). Typically, both the FTLM and the CTLM employ a solitary composite material dispenser that travels over the work surface onto which the composite material is to be applied. The composite material is typically laid down a single row (of composite material) at a time to create a layer of a desired width and length. Additional layers may thereafter be built up onto a prior layer to provide the lay-up with a desired thickness. FTLM's typically apply composite material to a flat transfer sheet; the transfer sheet and lay-up are subsequently removed from the FTLM and placed onto a mold or on a mandrel. In contrast, CTLM's typically apply composite material directly to the work surface of a mandrel. FTLM and CTLM machines are also known as automated tape laydown (ATL) and automated fiber placement (AFP) machines with the dispenser being commonly referred to as a tape head.
The productivity of ATL/AFP machines is dependent on machine parameters, composite part lay-up features, and material characteristics. Machine parameters such as start/stop time, course transition time, and cut/adding plies determine the total time the tape head on the ATL/AFP is laying material on the mandrel. Composite lay-up features such as localized ply build-ups and part dimensions also influence the total productivity of the ATL/AFP machines. Key material factors that influence ATL/AFP machine productivity are similar for a thermoset resin matrix composite when compared with a thermoplastic matrix composite yet there are a couple of key differences. For thermoset resin matrix composites, key factors are impregnation levels, surface resin coverage, and “tack”. “Tack” is the adhesion level necessary to maintain the position of the tape/tow on the tool or lay-up after it has been deposited on it. Thermoset resins are partially reacted and therefore “tack” is achieved through a combination of molecular diffusion between the two laminating surfaces and chemisorption between the polar, unreacted chemical moieties. Due to the unreacted nature of the thermoset resin, the ATL/AFP process is generally performed at room temperature but in humidity controlled rooms due to the moisture sensitivity on the tack level of the material.
Thermoplastic matrix composites have similar key factors as thermoset matrix composites for ATL/AFP machine productivity but since the thermoplastics polymer matrices are generally fully reacted in the tape it lacks “tack” at ambient conditions. The fully reacted thermoplastics generally have low surface energies making adhesion at room temperature unlikely. Furthermore, the high performance thermoplastic matrices are in their “glass” state at room temperature making the molecular diffusion mechanism for “tack” virtually impossible. Thus, “tack” is achieved in thermoplastic composites by dynamically applying additional energy in the form of thermal, ultrasonic, optical (laser), and/or electromagnetic (induction) to the lay-up and incoming tape to raise the temperature of the materials above their softening and/or melt temperature in order to facilitate molecular diffusion of the polymer chains to occur between the two surfaces. Once the polymer chains have diffused across the surface, the additional energy added to the materials needs to be removed to a level that will prevent distortion of the laminated lay-up once the lamination pressure from the ATL/AFP head is removed. This rapid flux of energy into and out of the lay-up makes it desirable from an energy usage and lay down speed to perform this process step at the lowest possible temperature and energy without compromising on the temperature performance of the resulting composite part.
Consolidation is typically necessary to remove voids that result from the inability of the resin to fully displace air from the fiber bundle, tow, or roving during the processes that have been used to impregnate the fibers with resin. The individually impregnated roving yarns, tows, plies, or layers of prepregs are usually consolidated by heat and pressure by compacting in an autoclave. The consolidation step has generally required the application of very high pressures and high temperatures under vacuum for relatively long times. Furthermore, the consolidation process step using an autoclave or oven requires a “bagging” operation to provide the lay-up with a sealed membrane over the tool to allow a vacuum to be applied for removal of air and to provide the pressure differential necessary to effect consolidation in the autoclave. This process step further reduces the total productivity of the composite part operation. Thus, for a thermoplastic composite it would be advantageous to in-situ consolidate to a low void composite while laminating the tape to the substrate with the ATL/AFP machine. This process is typically referred to as in-situ ATL/AFP and the material used in that process called an in-situ grade tape.
In general, thermoplastic composites have had limited success to date, due to a variety of factors including high processing temperatures (currently around 400° C.), high pressures, and prolonged molding times needed to produce good quality laminates. Most of the efforts have been focused on combining high performance polymers to structural fibers which has only exacerbated the process problems. Because the length of time typically required to properly consolidate the prepreg plies determines the production rate for the part, it would be desirable to achieve the best consolidation in the shortest amount of time. Moreover, lower consolidation pressures or temperatures and shorter consolidation times will result in a less expensive production process due to lowered consumption of energy per piece for molding and other manufacturing benefits.
Accordingly, the fiber-reinforced thermoplastic materials and methods presently available for producing light-weight, toughened composites require further improvement. Thermoplastic materials having improved process speeds on automated lay-up machines and lower processing temperatures, and having no autoclave or oven step would be a useful advance in the art and could find rapid acceptance in the aerospace and high-performance automotive industries, among others.